fix: use theme background for transparent text inputs

[tjaniczek]

Summary

In outlined mode, the background color of the label is derived from colors?.background in theme or the backgroundColor style. When backgroundColor style is passed as transparent that would affect the border cover as well, resulting in border & label overlapping in active state.

This PR switches background of the cover from transparent back to theme's background as a last resort.

Closes: #3759

Test plan

Tested on the example app + simple unit test added.

[github-actions]

The mobile version of example app from this branch is ready! You can see it here.

[ravithoria]

@tjaniczek facing issue, outline colour overwrite label. Here is full code:

"react-native-paper": "^5.12.3",

import { MD3LightTheme as DefaultTheme, PaperProvider } from 'react-native-paper';
const theme = {
    ...DefaultTheme,
    colors: {
        ...DefaultTheme.colors,
        primary: 'tomato',
        secondary: 'yellow',
        background: 'transparent',
    },
};

export default function Main() {
    return (
        <PaperProvider theme={theme}>
            <App />
        </PaperProvider >
    );
}

AppRegistry.registerComponent(appName, () => Main);

=====
Home.js
=====
import { TextInput, useTheme } from 'react-native-paper';
export default function Home({) {
    const theme = useTheme();
return (
        <View style={CommonStyles.container}>
        <ImageBackground
                style={{ flex: 1, backgroundColor: 'black' }}
                source={require('../assets/images/gradientBlue.png')}>
                <TextInput
                    placeholder='Email'
                    label="Email"
                    mode={'outlined'}
                    value={email}
                    onChangeText={email => setEmail(email)}
                    textColor='white'
                    outlineColor='white'
                    activeOutlineColor='white'
                    style={{fontFamily: fonts.BeVietnamProRegular}}
                     theme={{
                     colors: { onSurfaceVariant: 'white'} // label colour
                      }}
                       />
         </ImageBackground >
        </View >
)
}